PROJECT SUMMARY: There are approximately 1.5 million new acute myocardial infarction (MI) cases in the USA yearly. Approximately half of cell death is due to reperfusion injury during ischemia/reperfusion (I/R), for which there is no standard therapy. Designing novel therapies for reperfusion injury is therefore of major clinical importance. Interestingly, the ketone body, beta-hydroxybutyrate (?-OHB), whose level is increased in DM, is an endogenous histone deacetylase (HDAC) inhibitor. We recently showed that an FDA-approved HDAC inhibitor, SAHA, reduces infarct size by 40% when given at the time of reperfusion via the activation of autophagy. Similarly, acutely increased ?-OHB by IP injection or fasting before I/R reduces infarct size after cardiac I/R in rats. These studies suggest that ?-OHB has robust cardioprotective effects. However, how ?- OHB protects myocardium during I/R is not clear. Whether given ?-OHB at the time of reperfusion still protects reperfusion injury is also unknown. Furthermore, our preliminary data showed that: 1) in ST elevation MI (STEMI) patients, ?-OHB levels are elevated in the acute phase; 2) ?-OHB is a HDAC inhibitor in immortalized human cardiomyocyte, AC16 cells; 3) SAHA, an HDAC inhibitor, protects mitochondrial DNA (mtDNA) level, membrane potential, and reduces reactive oxygen species (ROS) during I/R. As an HDAC inhibitor, whether ?- OHB protects mitochondrial function similar as SAHA is unknown. If ?-OHB, an endogenous metabolite, protects the mitochondria, we will be able to decode the endogenous protective mechanisms and devise new therapies for cardioprotection during I/R. Hypothesis: ?-OHB attenuates I/R induced mitochondrial and contractile dysfunction when given at the time of reperfusion, through HDAC inhibition and autophagy. Specific Aims: 1. Determine whether ?-OHB protects mitochondrial structure and function in cardiomyocytes during I/R through autophagy. 2. Determine whether ?-OHB reduces infarct size and preserves cardiac function through autophagy and mitochondria protection in a mouse I/R model when given at the time of reperfusion. 3. Determine whether ?-OHB level is elevated in STEMI patients and whether ?-OHB protects mitochondrial structure and function in human nucleated cells in blood. Significance: After finishing the study, we will determine whether ?-OHB, a nature metabolite and an endogenous HDAC inhibitor, reduces infarct size at the time of reperfusion and if it is through protecting mitochondrial function by autophagy. This will enable us to develop novel therapies for reperfusion injury. 1